Roll-to-roll deposition apparatus with improved web transport system

ABSTRACT

A system for the continuous deposition of a semiconductor material onto one or more webs of substrate material which are advanced therethrough includes a web transport system having a plurality of web support assemblies. Each web support assembly includes a base having a primary support arm pivotally mounted thereto so as to be displaceable from a first position to a second position. The support includes a first biasing member in mechanical communication with the primary support arm. The first biasing member operates to impart a first biasing force to the primary support arm so as to move it from its first position to its second position. The support includes a dancer arm which is pivotally mounted to the primary support arm so as to be displaceable from a first position to a second position relative to the primary support arm. The system further includes a second biasing member in mechanical communication with the dancer arm. The second biasing member operates to impart a second biasing force to the dancer arm so as to move it from its first position to its second position. A roller is rotatably supported on the dancer arm. The roller is configured to engage a portion of the web. The web support assembly operates to maintain continuous contact between the roller and the moving web of substrate material as it passes through the deposition system.

FIELD OF THE INVENTION

This invention relates, generally, to roll-to-roll apparatus for thecontinuous deposition of semiconductor material onto a moving web ofsubstrate material. More specifically the invention relates toroll-to-roll systems which include assemblies for supporting and guidingan end portion of a vertically disposed, elongated web of substratematerial as it moves therethrough. In particular the invention relatesto a system for the continuous production of photovoltaic devices.

BACKGROUND OF THE INVENTION

Systems and methods have been developed for the high volume fabricationof semiconductor structures such as photovoltaic devices. In suchprocesses, one or more webs of substrate material, typically stainlesssteel, or composites, are continuously fed from a payout chamber througha plurality of deposition stations in which layers of semiconductormaterial are sequentially deposited onto the webs. The coated webs arethen wound onto rolls in a take-up chamber and removed for subsequentprocessing into photovoltaic modules. In particular systems of thistype, the web or webs of substrate material are maintained in a verticalorientation as they pass through the deposition apparatus. Some systemsof this type are shown, for example, in U.S. Pat. No. 4,423,701 and inpublished U.S. patent application 2004/0040506, which are incorporatedherein by reference.

Web transport and guidance are critical parameters in deposition systemsof this type. In order to assure deposition of high qualitysemiconductor layers, precise tolerances must be maintained between themoving substrate web and active components of the deposition system.Likewise, the moving web must travel along a precise path through thevarious chambers so as to avoid binding, twisting or other deformationswhich could compromise the integrity of the substrate web or thedeposition system. All of these problems are complicated by the factthat deposition systems of this type frequently have about 300 foot ormore lengths of substrate web actively moving therethrough at any time,and a typical substrate material has a weight of approximately ¼ poundper linear foot. The relatively heavy, relatively long substrate cancause problems of sagging and twisting since gravity tends to urge thesubstrate into a catenary configuration, and these problems can beenhanced by web geometry variation and differential thermal expansion ofthe web in the apparatus.

In apparatus of this type, transport systems for the substrate web mustalso be designed so as to avoid making contact with the semiconductormaterial deposited on the active face of the substrate web, since suchcontact could degrade the operational performance of the semiconductordevice. And finally, substrate transport systems must also be designedso as to avoid bending, burring or other deformation of the substrateweb, since such damage will prevent the coated substrate material frombeing wound into a uniform roll. This factor is critical since any suchnon-uniformity in the roll can damage the sensitive semiconductor layersduring subsequent processing. In one prior art approach as is disclosedin published U.S. patent application 2006/0278163 a substrate supportsystem incorporates a magnetic guidance assembly and a number of supportrollers to transport vertically oriented webs through a depositionsystem.

The rollers of the system in the '163 patent application guide anddirect edges of the moving web. However, in the operation of systems ofthis type it has been found that problems can arise because ofirregularities in the web material itself as a result of geometricvariations in the manufacture of the web and/or deformations resultantfrom differential thermal expansion. As a result of such variations, theamount of force exerted by the moving substrate web on any one rollermay vary as the web moves therethrough. Excessive force between theroller and the web can deform the edge of the web creating a burr,buckle or bend. Decreased force between the roller and the web can alsobe a problem, particularly if the web moves away from the roller since,under the high vacuum, relatively high temperature conditionsencountered in the deposition chamber, cold welding of the substrate androller can occur when contact is reestablished causing damage to the weband/or the roller. Therefore, there is a need for a web support systemwhich can operate to reliably and precisely move relatively heavy, longwebs of substrate material through a multistage high vacuum depositionsystem of the type used for the manufacture of photovoltaic devices andother semiconductor devices. Any such system should be relatively simpleand reliable in its operation and should be compatible with high vacuum,ultra clean conditions and should not introduce contamination into thesystem.

As will be explained in detail hereinbelow, the present inventionprovides high volume systems for the continuous preparation ofphotovoltaic and other semiconductor devices, which systems incorporatean improved web transport assembly. These and other aspects andadvantages of the present invention will be apparent from the drawings,discussion and description which follow.

SUMMARY OF THE INVENTION

The present invention comprises a system for the continuous roll-to-rolldeposition of a semiconductor material such as a photovoltaic material.The system includes a web transport for moving a web of substratematerial therethrough. The web transport includes an improved websupport assembly for supporting and guiding the web. The web supportassembly includes a base, a primary support arm which is pivotallymounted to the base so as to be displaceable from a first position to asecond position relative to the base, and a first biasing member whichis in mechanical communication with the primary support arm. The firstbiasing member is operable to impart a first biasing force to theprimary support arm so as to move it from its first position to itssecond position. The web support assembly further includes a dancer armwhich is pivotally mounted on the primary support arm so as to bedisplaceable from a first position to a second position relative to theprimary support arm. A second biasing member is in mechanicalcommunication with the dancer arm. The second biasing member is operableto impart a second biasing force to the dancer arm so as to move it fromits first position to its second position. The web support assemblyfurther includes a web support roller which is rotatably supported bythe dancer arm. The roller is configured to engage a portion of the web.In specific instances, the first biasing force is greater than thesecond biasing force. In particular instances, the biasing members maybe springs, clastomeric bodies, pneumatic cylinders, magnetic devices,hydraulic cylinders, and various combinations of the foregoing.

In a specific instance, the first biasing member is a spring whichextends between a primary support arm and the base. In some instances,the second biasing member may be a spring which extends between thedancer arm and the primary support arm.

In specific instances, the web support roller includes a groove which isconfigured to engage a web, and this groove may, in particularinstances, be an asymmetric groove. In some instances, the asymmetricgroove may be configured so that the two faces of the groove form a 90degree angle. In specific instances, the groove is configured so that anend surface of the substrate does not contact the base of the groove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a web support assembly of the presentinvention;

FIG. 2 is a perspective view of the web support assembly of FIG. 1showing a web of substrate material engaged therewith;

FIG. 3 is a perspective view of a primary support arm and a dancer armof the web support assembly of FIG. 1;

FIG. 4 is an illustration of the web support assembly of FIG. 1 in adown position wherein a high substrate load is imposed thereupon;

FIG. 5 is a depiction of the web support assembly of FIG. 1 in an upposition wherein a low level of contact force is imposed on thesubstrate; and

FIG. 6 is a cross-sectional view of a portion of a roller of the presentinvention as engaged with a web of substrate material.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a roll-to-roll deposition apparatuswhich includes an improved web transport system which includes a numberof web support assemblies which may be utilized to support and guide asubstrate material through a processing system. In that regard, theinvention will have utility in a large number of systems whereintransportation of a web of material is an aspect of the processingsystem. The present invention will be described with regard to aspecific utility wherein it is employed to guide and support a substrateweb in a system for the high volume deposition of photovoltaicsemiconductor material in a continuous roll-to-roll process.

The web support assemblies of the present invention are configured so asto maintain a portion of the web support in continuous contact with anedge of the substrate despite fluctuations in substrate width and/orposition. Furthermore, the web support assembly is configured so thatexcessive force between the edge of the substrate web and the contactedsupport is minimized. In exemplary embodiments, the web support assemblyincludes a multi-biasing arrangement configured to bias the substrateweb between a first position and a second position. In an exemplaryembodiment, the multi-biasing arrangement includes a biasing forcetransition zone with a discontinuous contact force at a web/supportcontact location. In an exemplary embodiment, a portion of the web incontact with the web support has a deflection that is substantially zerowithin the biasing force transition zone. In exemplary embodiments, theweb support assembly biases the web with a greater biasing force along afirst direction compared to a biasing force exerted upon the web along asecond direction.

The system of the present invention may be implemented in a number ofembodiments. One specific embodiment as particularly adapted for use inan apparatus for the fabrication of photovoltaic devices will be shownherein. In some embodiments, a first plurality of web support assemblieswill be utilized to support/guide a first edge of a substrate web, whileone or more web support assemblies are used to support/guide a secondedge of the substrate web. In certain embodiments, a portion of dieplurality of web support assemblies will be configured different fromthe other web support assemblies along the first and/or second edges ofthe web, for example, in terms of structural configuration,load/temperature capability, materials, etc. However, it is to beunderstood that other embodiments of the invention may be implemented,and likewise, the systems and apparatus of the present invention may beincorporated into systems utilized for the manufacture of a variety ofmaterials. It is to be further understood that the Figures of thisapplication are not drawn to scale; rather the Figures are drawn toillustrate most clearly the principles of this disclosure discussedherein.

Referring now to FIGS. 1 and 2, there is shown a web support assembly10, configured in accord with an embodiment of the present invention.The web support assembly 10 includes a base portion 12 which functionsto support the remainder of the assembly and further functions to allowthe assembly to be mounted to other structure. The base 12, as well asother portions of the web support assembly, can be fabricated frommaterials that withstand loads applied thereto and that do not degradeprocesses of the system. For example, it would not be desirable where amaterial of the web support assembly releases gases or other contaminatethat degrades the integrity of a semiconductor deposition process. Incertain embodiments, the assembly materials include aluminum, mild orhigh-strength steel, stainless steel, a high strength plastic, andcombinations thereof In the present embodiment unless otherwise notedherein, the majority of the assembly components are made from aluminum.A primary support arm 14 is pivotally mounted to the base 12 by a pivotjoint 16 which allows the primary support arm 14 to be movable from afirst position to a second position, relative to the base.

The web support assembly also includes a biasing member, in thisinstance a coil spring 18 which is mounted between the base 12 and theprimary support arm 14. The coil spring 18 operates to exert a biasingforce which tends to move the primary support arm from a first positionto a second position. In many embodiments, the spring 18 will bepreloaded a predetermined amount, for example, to urge the primarysupport arm upward as shown in FIG. 1 (e.g. second position), to suit aconfiguration of web (e.g. geometry and weight) and web movementparameters in the processing system. While tie biasing in the FIG. 1embodiment is accomplished by the coil spring 18, other biasing memberssuch as elastomeric bodies of material such as synthetic or naturalrubber may be used to provide the biasing force. Yet other biasingmechanisms known in the art such as gas filled pneumatic cylinders,hydraulic cylinders, counterweights, magnetic devices (includingelectromagnetic devices) and the like may also be used as biasingmembers. Also, while the spring 18 is shown as being in a particularlocation between the base 12 and support arm 14, it will be appreciatedthat it could be otherwise disposed. For example, the biasing may beaccomplished by a coil spring associated with the pivot 16.Alternatively, the biasing member need not be in any mechanicalcommunication with the base. For example, biasing may be accomplished bya spring or elastomeric body extending between the primary support arm14 and some other portion of the apparatus in which the system isdisposed.

The web support assembly 10 of FIGS. 1-3 further includes a dancer arm20 which is pivotally supported on the primary support arm 14, in thisinstance through a pivot joint 22. A second coil spring 24 is disposedbetween the dancer arm 20 and primary support arm 14 and it operates tobias the dancer arm 20 between a first and second position relative tothe primary support arm 14. As discussed with regard to the first spring13, the second spring 24 may be otherwise disposed; or, it may compriseanother type of biasing member such as an elastomeric body, pneumaticcylinder, hydraulic cylinder, counterweights, magnetic device or thelike. Rotatably affixed to the dancer arm 20 (at hole 28 shown in FIG.3) is a support roller 26, which operates to engage an edge of a web ofsubstrate material, and as such it may be grooved or otherwiseconfigured to aid in its retention of the web. In this embodiment, thesupport roller is primarily made of substantially non-magnetic, 304stainless steel. In an alternative embodiment, the support roller caninclude a ceramic material that contacts the web.

Referring now to FIG. 2, there is shown a perspective view of the websupport assembly 10 of FIG. 1 with a portion of a web of substratematerial 30 engaged therewith. It is to be understood that in a typicalapplication the substrate material 30 comprises an elongated web ofmaterial; however, for purposes of simplification, only a portion of theweb is shown in FIG. 2. Additionally, the embodiment of web supportassembly shown in FIG. 2 is configured so two webs can be independentlyguided/supported, wherein each roller supports/guides a web moving overthe roller independent from the other web-roller interaction, Forclarity purposes only one web is shown in phantom outline in FIG. 2, Itis to be understood that even though only one side (one web support) ofFIG. 2 has reference numerals and the same side is shown in otherFigures, the principles discussed herein with respect to the partsdenoted by reference numerals apply to the other web support withoutreference numerals in FIG. 2.

In the FIG. 2 illustration, the support roller 26 has engaged the edgeof the vertically disposed substrate 30, and as will be seen, a groove27 in the roller facilitates retention of the substrate 30. Referring toFIG. 4, the weight of the substrate 30 bearing onto the roller 26 hascompressed the second coil spring (24 shown in FIG. 1) between thedancer arm 20 and primary support arm 14. This is because the biasingforce of the second spring has, in this embodiment, been selected so asto be less than the biasing force of the first spring 18 and sufficientto overcome the weight of the dancer arm and roller. In the FIG. 2configuration the first spring 18 is supporting the primary arm 14, thedancer arm 20 and the roller 26 against the downwardly exerted force ofthe substrate 30. If that force increases because of distortions of thesubstrate web or the like, the spring 18 will absorb further downwardforce preventing damage to the web while maintaining good contact withthe web. Distortions of the substrate web can be caused, in non-limitingexamples, from web geometry variations, thermal and dynamic loads, etc.For example, if a portion of the bottom surface/edge of the substrateweb being supported by the roller is disposed below a theoretical passline of the web, the web will tend to exert a contact force upon theroller to move the roller toward a position shown in FIG. 4. Here thetheoretical pass line is a line of travel of the bottom surface/edge ofthe web if it was perfectly straight, or the web bottom surface passline has no distortions from that line due to part/web geometryvariation, thermal and dynamic loads, movement parameters, etc. In thisembodiment, the first spring is configured so a threshold predeterminedcontact force between the roller and the web is not exceeded. Thethreshold contact force can be based on deformation, web/rollerdeflection, material yield, etc. In this embodiment, the thresholdcontact force is based on a predetermined force that will not deform theweb, e.g. dent, crease etc., beyond a predetermined amount when the webis disposed below the pass line.

In that regard, FIG. 4 shows a side elevation view of the assembly 10 ina downwardly biased position. In the event that the web downward forcedecreases, the spring 18 will bias the primary support arm 14, theassociated dancer arm 20 and roller 26 back toward a neutral position.Should the web downward force still decrease, as would be the case ifthe web were moving so as to lift from the roller, e.g. when the bottomsurface of the web moves above the theoretical pass line, the spring 18will drive the primary support arm 14 to its full extent of travel as isshown in FIGS. 2, 3, 5; and at that point, the second spring 24 willoperate to lift the dancer arm 20 so as to further raise the supportroller 26 and thereby maintain contact with the edge of the web. Asnoted above, the biasing force of the second spring 24 is generally lessthan that of the first spring 18. Therefore, the pressure exerted by theroller onto the web will be less than it would be in a full downwardcontact position; but such pressure will be sufficient to maintain thetwo in contact and prevent the web from leaving the roller. The biasingforces of the springs can be selected based upon specifics of aprocessing system and a substrate web as will be discussed hereinbelow.In this embodiment, the first and second springs are configured toprovide a discontinuous transition of contact force between the rollerand web when the roller is in a force transition zone between the rollerpositions of FIGS. 4 and 5. Within that transition zone there is aposition where the vertical displacement of the roller is substantiallyzero and the force between the web and roller either increases ordecreases depending on the whether the web is moving toward a directionbelow or above the pass line. In the embodiment described hereinabove,the web support can be considered to bias the substrate web between aposition below the theoretical pass line, similar to that shown in FIG.4, and another position above the pass line, similar to that shown inFIG. 5.

The rollers used to contact and support/guide the web may be variouslyconfigured. In this embodiment, the roller includes a groove or someother feature for maintaining the substrate and roller in engagement.Referring now to FIG. 6, there is shown a portion of a roller 26 of thepresent invention having a web of substrate material 30 engaged with agroove 27 therein, as shown in FIG. 2. In the FIG. 6 embodiment thegroove is an asymmetric groove. That is to say, the two faces of thegroove have different angles of contact with the substrate 30. Thegroove has a first face 32 which contacts an edge of a first face orsurface of the substrate 30 at a first angle A, which in someembodiments is in the range of 5 degrees to 45 degrees. The grooveincludes a second face 34 which contacts the substrate 30 at a secondangle B which in some embodiments is in the range of 5 degrees to 30degrees. In particular instances, angle B is greater than the angle A.Furthermore, in this embodiment angle A and angle B are complementaryangles; that is to say, they add up to 90 degrees. In specificinstances, the system is configured so that the deposition of thesemiconductor material takes place on the face of the substrate 30,which defines angle A. Other angular relationships and orientations maybe implemented. It will also be seen that in the FIG. 6 illustration theface or the end surface of the substrate web 30 does not contact thebase of the groove. Contact between the roller 26 and the web 30 isestablished by a pinch between the faces 32, 34 of the roller and theedges or corners of the web. It has been found that this type of contactprovides desirable web support/guidance and further prevents damage tothe face or end surface of the substrate.

In systems for the deposition of semiconductor material onto a movingsubstrate web wherein the semiconductor deposition takes place on theface of the web closest to roller face 32, very good results are foundutilizing the geometric relationships of FIG. 6. In this regard, aroller configured as per FIG. 5 is very effective in maintaining thesubstrate 30 in a precise alignment with a deposition station whileavoiding damage to the substrate.

In view of the foregoing, various support assemblies may be readilyconfigured by those of skill in the art. The precise dimensions andcharacteristics of the assembly will depend upon particularapplications. As discussed above, a typical high volume continuousprocess apparatus for the deposition of semiconductor material mayemploy stainless steel substrate material having a weight ofapproximately 1 pound per linear foot. In one system of this type, thesubstrate material is supplied in coils of approximately 8000 feet inlength, and at any one given time approximately 279 feet of thesubstrate web is actively moving through the deposition system. And asis further noted, such systems may include up to 6 webs movingtherethrough in a substantially parallel relationship. In thisconfiguration, each web has 60 supports associated therewith. Therefore,each roller will be supporting approximately 1.16 pounds on the average.In a system of this type, the primary spring (18 hereinabove) willtypically be preloaded to a level of approximately 2.5 to 3.5 pounds andthe dancer spring (24 hereinabove) will be selected so that a load of0.75 to 1 pound will compress it to its fully down position, as shown inFIG. 4. In instances where differently configured systems and/orsubstrate webs are employed, the biasing force of the springs or otherbiasing members may be adjusted accordingly.

The web support assembly of the present invention has been describedwith regard to its use to support and guide the bottom edge of avertically disposed substrate. However, assemblies of this type may alsobe disposed to engage a top edge of a vertically disposed substrate. Insuch instances, the biasing forces may be adjusted accordingly. Also,the support assembly has been described with regard to its use inconjunction with steel substrates. It is to be understood that suchassemblies may be used to support and guide other types of substratewebs, including polymeric substrates, composite substrates, fibroussubstrates and the like.

In view of the teaching presented herein, other modifications,variations and embodiments of this invention will be apparent to thoseof skill in the art. All of such embodiments are within the scope ofthis invention. The foregoing drawings, discussion and description areillustrative of specific embodiments of the invention, but are not meantto be limitations upon the practice thereof. It is the following claims,including all equivalents, which define the scope of the invention.

1. In a roll-to-roll system for the continuous deposition of asemiconductor material onto a web of substrate material, wherein in theoperation of said apparatus, the web of substrate material iscontinuously advanced therethrough, wherein said system includes aplurality of web support assemblies which engage and direct said web asit passes through said system, each web support assembly comprising: abase; a primary support arm which is pivotally mounted on the base so asto be displaceable from a first position to a second position relativeto said primary support arm; a first biasing member in mechanicalcommunication with said primary support arm, said first biasing memberbeing operable to impart a first biasing force to said primary supportarm so as to move said primary support arm from its first position toits second position; a dancer arm which is pivotally mounted on saidprimary support arm so as to be displaceable from a first position to asecond position relative thereto; a second biasing member in mechanicalcommunication with said dancer arm, said second biasing member beingoperable to impart a second biasing force to said dancer arm so as tomove said dancer arm from its first position to its second position; anda web support roller rotatably supported by said dancer arm, said rollerbeing configured to engage a portion of said web.
 2. The system of claim1, wherein in said web support assembly, said first biasing force isgreater than said second biasing force.
 3. The system of claim 1,wherein in said web support assembly, said first biasing member and saidsecond biasing member are selected from the group consisting of:springs, elastomers, pneumatic cylinders, hydraulic cylinders, magneticdevices, and combinations thereof.
 4. The system of claim 1, wherein insaid web support assembly, said first biasing member is a spring whichextends between said primary support arm and said base.
 5. The system ofclaim 1, wherein in said web support assembly, said second biasingmember is a spring which extends between said dancer arm and saidprimary support arm.
 6. The system of claim 1, wherein in said websupport assembly, said web support roller includes a groove which isconfigured to engage an edge of the web.
 7. The system of claim 6,wherein said groove is an asymmetric groove.
 8. The system of claim 6,wherein said groove includes a first face and a second face, and whereinsaid first face and said second face form a 90 degree angle.
 9. Thesystem of claim 6, wherein said groove is configured so that said grooveincludes a first face and a second face, and wherein said facesestablish a pinch contact with said substrate so that an end surface ofsaid substrate does not contact the bottom of said groove.
 10. Thesystem of claim 6, wherein said roller is configured so that when web ofsubstrate material is disposed within the groove, a first face of saidgroove forms a first angle A, with a first face of said web, and asecond face of said groove forms a second angle B with a second face ofsaid web, and wherein angle A and angle B are different.
 11. The systemof claim 10, wherein angle A and angle B are complementary.
 12. Thesystem of claim 1, wherein said web of substrate is continuouslyadvanced through said system in a vertical orientation and wherein atleast some of said plurality of web support assemblies engage a loweredge of said web.
 13. The system of claim 12, wherein at least one ofsaid plurality of web support assemblies engage an upper edge of saidweb.
 14. The system of claim 12, wherein at least one of the pluralityof web support assemblies has a different configuration compared to theother web support assemblies.
 15. The system of claim 1, wherein in theoperation of said system, a plurality of webs of substrate material arecontinuously advanced therethrough, and wherein said web supportassemblies engage and guide said plurality of webs as they pass throughsaid system.
 16. The system of claim 15, wherein the system comprises afirst web support assembly and a second web support assembly configuredso a first web and a second web of the plurality of webs have a spacedsubstantially parallel relationship.
 17. The system of claim 16, whereinthe web support roller of the first web support assembly is configuredto support/guide the first web independent of the second web-rollerinteraction with respect to the second web support assembly.
 18. Thesystem of claim 17, wherein the first and second web support assembliesare joined to a common base.
 19. The system of claim 1, wherein saidsystem is operable to deposit a photovoltaic semiconductor material ontosaid web of substrate material.
 20. In a roll-to-roll system for thecontinuous deposition of a semiconductor material onto a web ofsubstrate material, wherein in the operation of said apparatus, the webof substrate material is continuously advanced therethrough, whereinsaid system includes a plurality of web support assemblies which engageand direct said web as it passes through said system, each web supportassembly comprising a multi-biasing arrangement configured so a surfaceof the web is biased along a first direction with a first bias force andthe surface of the web is biased along a second direction with a secondbias force, the second direction being substantially opposite the firstdirection.
 21. The web support assembly of claim 16, wherein themulti-biasing arrangement is configured to have a biasing forcetransition zone between a web first position and a web second positionwhere the web has a deflection within the biasing force transition zonethat is substantially zero.
 22. The web support assembly of claim 16,wherein the multi-biasing arrangement biases the surface of the web witha greater force along the first direction compared to a biasing forceexerted upon the web along the second direction.
 23. The web supportassembly of claim 18, further comprising a roller, wherein the firstbias force along the first direction reacts web movement into theroller, and the second bias force along the second direction acts tomaintain contact between the roller and web as the web moves away fromthe roller.
 24. The web support assembly of claim 16, wherein thebiasing arrangement includes a coil spring.